My invention provides a liquid detector system for use with a tank holding a liquid propellant in a zero gravity environment. In particular, a liquid detector system is provided for determining the quantity of liquid propellant remaining in a spacecraft propellant tank.
In general, it has been difficult to be able to place a replacement satellite into service within a week of the last possible moment without also risking interruption of service due to the end-of-life of the original satellite. In short, it was difficult to predict, with any certainty, how much liquid propellant was remaining in an existing satellite propellant tank in order to send up into outer space the replacement satellite at the last possible moment in order to maximize efficiency.
U.S. Pat. No. 3,161,050 (Exner) discloses a liquid level apparatus for measuring the height of a liquid in a container. The apparatus comprises an elongated heater and a pair of spaced thermocouples which are mounted on a movable member that moves in a track on the outside of the container along the length of the heater. In operation, the heater is continuously energized and the thermocouples respond to the temperature difference in the liquid wall adjacent to the heated zone to indicate the liquid interface.
U.S. Pat. No. 3,465,315 (Alexander et al.) discloses a liquid level indicator device comprising a stainless steel tube provided with a thermocouple secured to a disc and a heater element positioned adjacent to the thermocouple. The liquid level indicator operates by closing a switch to apply power from a battery to the heater element and measuring the temperature through a thermocouple by a liquid level indicator. When the liquid is in contact with the thermocouple/disc, the temperature indication will appear as 4A as seen in FIG. 5 of the '315 patent. When the liquid recedes from the thermocouple, the indicating device will provide a temperature indication 4B as seen in FIG. 5 of the '315 patent.
However, for a zero gravity application, constant heat applied as in Exner and Alexander et al. would yield no evidence of liquid, due to the displacement of the liquid and the creation of a gas barrier at the tank wall thereby making it appear that no liquid is present. Thus, the above-described prior art devices are continuous units and are designed to find a moving interface in the normal 1-g earth atmosphere and will not work in a zero-g environment.
U.S. Pat. Nos. 3,196,679, 4,449,403, and 4,592,230 are of background interest with respect to the present invention.